1. Field of the Invention
The present invention relates to a piston ring utilized for an internal combustion engine, and more particularly, relates to a piston ring capable of effectively preventing adhesion (deposition) of aluminum to a piston ring.
2. Related Art
Generally, a compression ring and an oil ring are mounted as set of a piston ring to a piston that performs reciprocating motion. The compression ring has a function of preventing a blow-by phenomenon in which highly pressurized combustion gas is blown out from a combustion chamber side to a crank chamber side. On the other hand, the oil ring is used for mainly achieving a function of suppressing an oil-up phenomenon in which an excessive lubricant adhering on a cylinder inner wall surface invades into the combustion chamber side from the crank chamber side and then is consumed. In a conventional standard combination of the piston ring, there is well known a combination of three rings comprising two compression ring including a one top ring and a second ring and one oil ring.
In recent, according to an increasing tendency of light weight and high outputting of an internal combustion engine, it has been highly required for a piston ring to have a highly improved quality. In a conventional technology for improving durability of the piston ring of the internal combustion engine, wear resistance surface treatment such as nitriding treatment, ion plating treatment, or hard chromium plating treatment has been proposed.
In these surface treatments, the nitriding treatment has been highly evaluated and widely used as surface treatment for the piston ring, which is utilized under harsh operating conditions, because the nitriding treatment provides an excellent wear resistance performance.
However, although the piston ring effected with the nitriding treatment is excellent in the wear resistance, when it is applied to a piston made of an aluminum alloy, wear of a ring groove of the piston tends to be increased. In addition, as shown in FIGS. 1A to 1C, a phenomenon of adhering aluminum to a lower surface 3 of a piston ring 1 from a lower surface of a groove of a piston made of aluminum alloy has been caused (see FIG. 1C).
FIG. 2A to 2C are charts showing change of surface conditions of upper and lower surfaces 2 and 3 of the ring groove of the piston carried out by using a contact stylus surface-roughness testing machine. As shown in FIG. 2, the surface conditions of the upper and lower surfaces 2 and 3 of the piston ring groove changes from a normal condition shown in FIG. 2A to an aluminum adhesion condition shown in FIG. 2C through a piston groove rough condition 2B.
In FIGS. 2A to 2C, the axis of abscissa represents a position of the piston and the axis of ordinate represents waviness of the piston groove, and in these figures, letter F denotes a front direction, AT denotes anti-thrust direction, R denotes rear direction and T denotes thrust direction.
Furthermore, FIGS. 3A to 3C show aluminum adhesion mechanism, in which FIG. 3A shows a state that the lower surface 3 of the piston ring 1 and a groove lower surface 11 of an aluminum alloy piston 10 contact each other through oxide films 8, each having thickness of less than 0.2 μm, formed on both the surfaces, FIG. 3B shows a state that a stress of the contacting oxide films 8 becomes locally high, which destroys the oxide films 8, and Fe on the lower surface 3 of the piston ring 1 and Al on the groove lower surface 11 of the aluminum alloy piston 10 are joined, and FIG. 3C shows a state that an aluminum alloy 20 is fused to the lower surface 3 of the piston ring 1.
Further, FIG. 4 shows an enlarged view of the aluminum adhering portion, and in FIG. 4, reference numeral 20 denotes the adhering aluminum and reference numeral 21 denotes a joined portion of the Al and Fe.
As mentioned above, according to vertical motion of the piston ring, when local wear resulted from this welding phenomenon occurs to a predetermined portion of the piston groove, sealing performance of the internal combustion engine is degraded by the blow-by of a blow-by gas, and hence, an output is reduced. This phenomenon occurs for a short time on the lower side of the piston ring groove and largely affects on the durability of the internal combustion engine. Accordingly, there have been conventionally proposed countermeasures to wear of the piston groove.
For example, as the countermeasure to the wear of the piston groove, in order to prevent the piston and the piston ring from directly contacting each other, the piston is subjected to anode oxidation film coating treatment, plating treatment or matrix strengthening treatment in the piston, and on the other hand, the piston ring is subjected to phosphor salt film coating treatment or plating treatment, or as shown in FIGS. 5A and 5B, resin coating treatment is applied to the surfaces of the piston 10 and piston ring 1 (for example, Defric (made by KAWAMURA KENKYU-SHO) coating treatment.
Furthermore, in order to solve the above problem, there has been developed and provided a piston ring in which an wear resistance treated layer such as nitride layer or chromium plating layer is formed to the upper and lower surfaces, or only lower surface of the piston ring, and a polybenzoimidazol resin film containing solid lubricant is formed on a surface of the wear resistance treated layer (see Patent Publication 1: Japanese Patent Application Laid-open Publication No. HEI 7-063266)
Moreover, there have also been developed and provided, by persons other than the applicant of the subject application, a piston ring having a surface which is coated with an heat-resistant resin containing a solid lubricant (see Patent Publications 2 and 3: Japanese Patent Application Laid-open Publication Nos. HEI 10-246149 and HEI 11-246823)
However, the above-mentioned countermeasures to the wear of the piston groove in the conventional technology may be effective for preventing adhesion of aluminum in an initial stage of the using of the piston, but are insufficient for a long term use of the piston, thus requiring improvement of durability.
More specifically, for example, although the above Patent Publication 1 discloses a surface coating film composed of polybenzoimidazol resin and solid lubricant (graphite or MOS2). The polybenzoimidazol resin is liable to be oxidized in its liquid resin at the formation of the coating film and is deteriorated with age. Thus, it is necessary to pay attention to use. In addition, the polybenzoimidazol is sometime difficult to stably maintain its quality for a long time.
Furthermore, the above Patent Publication 2 discloses a surface coating film composed of polyamideimide resin or polyimide resin and a solid lubricant (graphite, MOS2, WS2 or polytetrafluoroetylene). However, such surface coating film does not sufficiently prevent an aluminum adhesion and also involves cost increasing problem.
Still furthermore, the above Patent Publication 3 discloses a surface coating film composed of polyamideimide resin or polyimide resin and a solid lubricant such as MoS2 or Sb2O3. However, such coating film cannot sufficiently prevent the adhesion of aluminum, and Sb2O3 is harmful for environment, thus being difficult to use.